1. Field of the Invention
The present invention relates to a vacuum-brazing aluminum cladding material consisting of an aluminum or aluminum alloy core member and aluminum alloy clads covering at least one of opposite surfaces of the core member, and more particularly to such vacuum-brazing aluminum alloy clads which are suitably used for producing a hollow structure with high vacuum brazing stability, and with reduced amount of Mg vaporizing in a brazing furnace.
2. Discussion of the Prior Art
A vacuum brazing process is known as one method of brazing of Al or Al alloy materials or workpieces, without a brazing flux. The vacuum brazing process uses as a filler metal an Al-Si brazing alloy which contains about 1.0-2.0% of Mg, which is heated in a brazing chamber or furnace under vacuum or at a sub-atmospheric pressure. This vacuum brazing process is widely practiced for fabricating a hollow structure or assembly such as a heat exchanger. The vacuum brazing process is generally recognized as a brazing process wherein Mg contained in the brazing alloy and vaporized in the vacuum brazing furnace functions as a getter to absorb residual oxidizing gases in the furnace, and also destroys an oxide film on the surface of the brazing alloy upon vaporization of Mg, whereby the aluminum or aluminum alloy workpieces can be brazed together under vacuum with the brazing alloy. Usually, the vacuum-brazing alloy is provided in the form of a cladding layer or layers which cover one or both surfaces of an aluminum alloy core member of a brazing sheet consisting of an aluminum alloy of the JIS-(Japanese Industrial Standard) A-3003, for example. That is, the brazing sheet consists of the core member and the cladding alloy layer or layers covering the core member. The brazing sheet does not require any special pre-brazing treatment, namely, simply requires a degreasing treatment before the brazing sheet is used.
However, the vacuum brazing process utilizing such conventional brazing sheet suffers from potential drawbacks as described below.
First, the Mg component vaporized from the brazing sheet at the elevated brazing temperature tends to be deposited and accumulated on the wall of the brazing chamber or furnace, and the deposited Mg component absorbs oxidizing gases when the furnace is exposed to the atmosphere. When the brazing furnace is again heated, the absorbed oxidizing gases are released from the deposited Mg component, deteriorating the brazing condition in the furnace. Consequently, the deposited Mg component should be removed from the furnace at regular intervals. In practice, it is necessary to accomplish a small-scale cleaning of the brazing furnace on a routine basis before each brazing job, a medium-scale cleaning every several weeks of the brazing operation, and a large-scale cleaning involving disassembling of the furnace, one or two times each year. This cleaning procedure is cumbersome and lowers the brazing efficiency.
While the need for the cleaning operation of the brazing furnace may be reduced by reducing the content of the Mg component contained in the brazing alloy, it is difficult to reduce the Mg content without deteriorating the brazability of the brazing alloy, since the sufficient brazing stability may be obtained with the Mg content being normally about 1.5% and at least 1.0% under any condition. It is considered that the required Mg content of the brazing alloy may be more or less reduced by placing a Mg or Mg alloy ingot as a getter in the brazing furnace. However, the use of the Mg ingot as the getter actually results in increasing the overall amount of the Mg component vaporized in the brazing furnace.
A second drawback of the vacuum brazing process using the conventional brazing sheet occurs when the brazing sheet is used for manufacturing a hollow structure or assembly such as a drawn cup type of heat exchanger or radiator tank, which has joints formed in brazing on both inner and outer sides of the structure. That is, the vaporizing rate of the Mg component of the ordinary conventional brazing sheets is different on the inner and outer sides of the hollow structure to be produced, whereby the brazability is deteriorated, in particular, on the inner side of the hollow structure. Although reducing the Mg content of the brazing alloy is effective to improve the brazability on the inner side of the hollow structure to be produced, it is difficult to reduce the Mg content while maintaining the Mg content within a range which assures sufficient brazability on the outer side of the structure.
To improve the brazability in producing a hollow structure using a vacuum-brazing aluminum alloy, Publication No. 58-54909 of examined Japanese Patent Application proposes a brazing sheet in which the Mg content of the brazing alloy for the inner side of the structure is controlled within a range between 0.2% and 1.2%. Further, laid-open Publication No. 59-85364 of unexamined Japanese Patent Application proposes a brazing sheet in which the Mg content of the brazing alloy for the inner side of the hollow structure is zeroed. However, the two proposed brazing sheets indicated above still suffer from low brazability on the outer side of the hollow structure, since a portion of the brazing alloy for the inner side flows to the outside of the structure, at each joint between the adjacent brazing sheets which bounds the hollow inside and the outside of the structure.
The above proposed brazing sheets should be correctly oriented with respect to each other so that the brazing alloy clad layers on the brazing sheets are suitably positioned in relation to the inside and outside of a hollow structure to be produced. This orientation requirement makes it difficult to control the inventory of the brazing sheets and apply the brazing sheets to an automated brazing line. Therefore, the proposed brazing sheets do not provide a practically effective solution to the conventionally experienced problem.